Rear projection is an arrangement in which the viewer sees an image on a screen, the image of which is provided by projecting light from behind the screen through means such as a source projector located behind the screen. The light from the source projector is thus directed toward the audience, and the screen material serves to scatter the light so that it appears to come from all points on the screen and not directly from the projector. When projecting onto a three-dimensional wrap-around screen, the image is formed on the surface of a translucent diffusion layer which may be bonded to a rigid transparent substrate. Rear projection screens are available in rigid and nonrigid configurations. Suitable materials for the substrate material used in rigid screens include acrylic, polyethylene terephthalate, and polycarbonate plastics, as well as glass.
The light scattered from the screen towards the viewer is referred to as forward-scattered light. However, with any rear screen material some portion of the light will also be scattered back in the direction towards the projector. This is referred to as back-scattered light. The back-scattered light contributes nothing to the image formed on the screen, and a good rear projection screen is designed to minimize this waste of light.
In the case of three-dimensional wrap-around screens, the back-scattered light creates an additional problem. For example, a spherical or cylindrical screen may have wrap-around projection provided by a source projector utilizing a simple fisheye lens, although other optical elements can be employed to provide wide-angle image distribution. The back-scattered light from one portion of the screen will impinge on opposite portions of the screen and degrade the desired image with spurious diffuse light. The result is a loss of contrast, where contrast is the ability to accurately reproduce and differentiate light and dark areas of an image. Ambient light can also pass through the viewing side of a wrap-around screen and impinge on the opposite portions and impair the ability of the screen to perform well under lighting conditions typical to audience areas in places such as museums, office lobbies, and visitor centers.
The image displayed on a screen by a source projector will always be limited in contrast because a fully black portion of the image will have some small illumination under the best of conditions due to the nature of the source projector technology. Nonetheless, the contrast of images transmitted by the source projector can be very high. For example, a fully lit portion of the image can be 1000 times brighter than a nominally-unilluminated portion of the image, providing a contrast ratio of 1000:1. If a source projector transmits an image onto a rear screen in a totally dark room, and 20 percent of the light is back-scattered on to other portions of the screen, the contrast on the screen could be degraded to a ratio of about 4:1, depending upon the distribution of light in the image. Ambient light contributed by rooms that are not totally dark will further reduce contrast by illuminating the screen image from the audience side as well as adding to the spurious light inside the screen.
Generally, three-dimensional rear projection screens encompass a thin rear screen material that is supported by a rigid transparent substrate such as clear acrylic. In practice, screen contrast is often improved by darkening the rear screen material with a neutral gray dye or pigment. In such arrangements, the forward-scattered light passes completely through the darkened film and is reduced in intensity by a certain amount while the back-scattered light has, on the average, passed through half of the film thickness, and is reduced by about half of the same amount (the differences being attributable to light scattering within the film). Therefore, this kind of arrangement still suffers significantly from a loss of contrast caused by the impingement of back-scattered light on opposite portions of the screen.